As the Watt report (posted above) states, there's no way to collect physical evidence after so much time (especially when houses have been repaired). And it works both ways. If AK-47 shell casings were found in or near the houses, there's no way to prove whether the bullets were fired at Marines on Nov. 19th.
That's why I think NCIS agent did their usual thing and intimidated the hell out of Kilo Company Marines until someone was willing to play ball and say he heard someone "confess". Remember, that's what they tried in the Lt. Phan case and it blew up in their faces. Lucky for them, the Secretary of the Navy denied an investigation into their conduct.